Irregular satellite

In astronomy, an irregular moon, irregular satellite or irregular natural satellite is a natural satellite following a distant, inclined, and often eccentric and retrograde orbit. They have been captured by their parent planet, unlike regular satellites, which formed in orbit around them.

113 irregular satellites have been discovered, orbiting all four of the giant planets (Jupiter, Saturn, Uranus and Neptune). The largest of each planet are Himalia of Jupiter, Phoebe of Saturn, Sycorax of Uranus, and Triton of Neptune. In 1997, the first two Uranian irregulars were discovered, Caliban and Sycorax. It is currently thought that the irregular satellites were captured from heliocentric orbits near their current locations, shortly after the formation of their parent planet. An alternative theory, that they originated further out in the Kuiper belt, is not supported by current observations.

There is no widely accepted precise definition of an irregular satellite. Informally, satellites are considered irregular if they are far enough from the planet that the precession of their orbital plane is primarily controlled by the Sun.

In practice, the satellite's semi-major axis is compared with the planet's Hill sphere (that is, the sphere of its gravitational influence) $r_{H}$ $r_{H}$ . Irregular satellites have semi-major axes greater than 0.05 $r_{H}$ $r_{H}$ with apoapses extending as far as to 0.65 $r_{H}$ $r_{H}$ . The radius of the Hill sphere is given in the adjacent table.

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